To understand the cellular basis for the protective immune responses required for the resolution of bacteremia, we have been studying the murine model of Borrelia hermsii infection. Using this experimental system we have identified a unique role for B1b cell subset in protective immunity. Specifically we found that B1b cells expand concurrent with the resolution of B. hermsii bacteremia and the expanded cells persist in convalescent mice for a remarkably long time and contribute to a novel T cell-independent B cell memory. This seminal finding has been recapitulated using several important human pathogens. Interestingly, the antigens recognized by B1b cells in these bacterial infection systems include not only polysaccharides, but also bacterial outer-membrane proteins. Although the power of murine models in elucidating immunological mechanisms is unquestioned, it is not known whether a functional equivalent of B1b cell-mediated immunity exists in humans. Recently, a novel population of B cells (CD19+CD20+CD27+CD43+CD70-CD69-) was identified in human umbilical cord blood and adult peripheral blood that has several key characteristics of murine B1 cells. To investigate the function of human B1 cells, we have utilized CD34+ human hematopoeitic stem cell (HSC)-engrafted NOD/SCID/IL-2R??null (NSG) mice, referred to as Human Immune System mice (HISmice). We have found that HISmice generate several subsets of human B cells including B1 cells suggesting that the murine microenvironment is capable of supporting the development of human B1 cells from human HSCs. We also found that B. hermsii infection in HISmice results in recurrent episodes of bacteremia, a hallmark of relapsing fever. Moreover, the resolution of the primary episode of bacteremia was concurrent with the generation of a B. hermsii-specific human IgM response. In the murine infection model, B1b cells mount a specific antibody response to Factor H binding protein A, (FhBA), an outer-membrane protein of B. hermsii. Interestingly, IgM from B. hermsii-infected individuals or from B. hermsii-infected HISmice display an identical FhBA reactivity. Depletion of B cells by anti- human CD20 in HISmice results in loss of B. hermsii- and FhBA-specific antibody responses and in persistent bacteremia. Together, these findings suggest that functional equivalents of murine B1b cells exist in humans and HISmice. Although the characteristics of B. hermsii infection in HISmice mirror those of human relapsing fever, it is not known which subsets of human B cells are functionally responsible for controlling B. hermsii. The Aims of this proposal are to: (1) identify the human equivalents of murine B1b cells in HISmice and (2) to determine whether expanded human B cell population in HISmice persist and confer a heightened protective response to B. hermsii. Investigating the basis for protective TI responses conferred by B cell subsets in HISmice will not only validate our current understanding of B1b cells based on murine models, but will also serve as a translational platform, which can help us to explore the potential role for B1b cells in a number of bacterial infections in humans.